The long term goal of our research is to characterize the cellular and molecular mechanisms that endow an organism with its ability to adapt to changes in the eternal environment. In particular, we propose to determine how the marine mollusc Aplysia californica makes a switch between two types of related, but distinctly different, rhythmic behaviors. We have developed a hypothetical model that postulates that switches from one behavior to the other result from the phasic activity of sensory neurons. These sensory neurons contain both primary neurotransmitters and modulatory peptide cotransmitters. We propose that both types of neurotransmitters exert actions that are essential for the maintenance of proper phase and amplitude relationships between elements of the central pattern generator when behavioral transformations are made. Our work is, therefore, generally relevant to studies of plasticity in any rhythmic behavior that must accommodate changes in the external environment in a coordinated fashion. Additionally, we postulate a novel conceptualization of the physiological significance of the presence of primary transmitters and cotransmitters in neurons triggering switches from one behavior to another. We suggest that sensory neurons release two types of neurotransmitters because the firing patterns of neurons utilized during more then one phase of behavior is adjusted. Primary neurotransmitters adjust the activity of neurons coactive with sensory neurons, modulatory neurotransmitters adjust the firing patterns of neurons utilized after sensory neurons have stopped firing. Actions of modulatory neurotransmitters, are, however, phase-dependent. They are manifested on the phase of behavior that immediately follows sensory neuron activity, but they do not last long enough to continuously modify neural activity. We propose, therefore, to characterize a novel role for peptide cotransmission in a neural circuit undergoing a transformation from one rhythmic output to another. This data will help guide future studies of the role of neuromodulation in circuit selection, and may provide insights into dysfunctions of the nervous system that occur when cognitive processes necessary for the proper choice of behavior are perturbed, as is the case with obsessions and compulsions.